Turbo Coded Terrestrial Mobile Video Broadcasting

(1)

C.S. Lee, T. Keller, L. Hanzo

Dept. of Electr. and Comp. Sc., Univ. of Southampton, SO17 1BJ, UK.

Tel: +44-703-593 125, Fax: +44-703-593045

Email: [email protected]

http://www-mobile.ecs.sot on.a c.u k

ABSTRACT

Inthiscontributionweinvestigatedtheperformance

ofturbo-codedMPEG-2 compressionbased

terres-trial mobile video broadcasting. Our experiments

suggestedthatnon-hierarchical,iesingle-class

pro-tection of the MPEG-2 video stream exhibited a

similarerrorresiliencetotwin-classdata

partition-ing, since a high proportion of relatively sensitive

videobitshadtoberelegatedtothelowerintegrity

subchannel,wheninvokingapowerfullow-rate

chan-nelcodec in the high-integrity protection class for

thesake ofensuring theerror-free reception ofthe

moresensitive MPEG-2video bits,suchas control

headers,etc.

1. BACKGROUNDANDMOTIVATION

Inrecent yearsthreeDVB standardshaveemergedin

Eu-rope for terrestrial [1 ], cable-based [2 ]and

satellite-orien-ted [3 ]delivery of DVB signals. Themore hostile

propa-gation environmentof the terrestrial systemrequires

con-catenated Reed-Solomon [4, 5] (RS) and rate compatible

puncturedconvolutionalcoding[4 ,5](RCPCC)combined

withOrthogonalFrequencyDivisionMultiplexing(OFDM)

based modulation [6 ]. By contrast, the more benign

ca-bleandsatellitebasedmediafacilitatestheemploymentof

blind-equalisedmulti-levelmodemsusingupto256

quadra-tureamplitudemodulation(QAM)levels[6 ]. Theseschemes

arecapableofdeliveringhigh-denitionvideoatbitratesof

upto20Mbits/s instationarybroadcast-modedistributive

wirelessscenarios.

Themotivationofthiscontributionisto

investi-gatetheperformanceimprovementsattainableupon

invoking powerfulturbocodinginsteadofthe

stan-dardconvolutionalcodingandtoassessthesystem's

performanceinmobile,ratherthanstationary

prop-agationscenarios. Lastly,the potential ofinvoking

twin-classerrorprotectionisappraised. Ourresults

showedthatwiththeadventofturbocodingfor

ex-ample16QAMcanreplace4QAMwithoutdramatic

Thenancialsupportofthefollowingorganisationsis

grate-fullyacknowledged: EPSRC,UK inthe frameworkof the

con-tractGR/K74043;theEuropeanCommission;

ICT'2000,22-25May,Acapulco,Mexico

Area: Videocodinganddistribution

complexity and power increases, while allowing us

todouble the video bitrate and hence improve the

video quality.

2. DVB TERRESTRIALSCHEME

TheblockdiagramoftheDVBterrestrial (DVB-T)

trans-mitter [1 ] is shown in Figure 1, which is constituted by

an MPEG-2 video encoder, channel coding modules and

anOrthogonalFrequencyDivisionMultiplex(OFDM)

mo-dem[6 ,7]. Duetothepoorerrorresilience ofthe

MPEG-2 video codec, strongconcatenated channelcodingis

em-ployed,consistingofashortenedReed-SolomonRS(204,188)

outercode[4 ],whichcorrectsuptoeighterroneousbytesin

ablockof204bytes,andahalf-rateinnerconvolutional

en-coderwithaconstraintlengthof7[4 ,5]. Theoverallcode

ratecanbeadaptedbythevariablepuncturer,which

sup-portscoderatesof1=2(nopuncturing)aswellas2=3,3=4,

5=6,and7=8. Theparametersoftheconvolutionalencoder

aresummarisedinTable1. Ifonlyoneofthetwobranches

ofthetransmitterinFigure1isutilised,theDVB-Tmodem

issaidtobeoperatinginitsnon-hierarchicalmode.Inthis

mode,themodemcanhaveachoiceofQPSK,16-QAMor

64-QAMmodulationconstellations.

Rate 1=2

ConstraintLength 7

k 1

n 2

Polynomials(octal) 171,133

Table 1: ParametersoftheCC(n,k,K) convolutionalinner

encoderintheDVB-Tmodem.

Asecondvideobitstreamcanalsobemultiplexedwith

therstonebytheinnerinterleaver,whentheDVBmodem

isinitsso-calledhierarchicalmode[1 ]. Thechoiceof

mod-ulationconstellationsinthismodeisbetween16-QAMand

64-QAM. We shall be employingthis transmission mode,

whentheso-calleddatapartitioningschemeisusedtosplit

theincomingMPEG-2videobitstreaminto twoclassesof

data,withoneclasshavingahigherprioritythantheother

one. Thehigher priority datawill be multiplexedto the

most signicant bits (MSBs) of the modulation

(2)

signi-Mapper

OFDM

Inner

Interleaver

block

4/16/64 QAM

2K mode

channel

RS(204,188)

Outer

conv.

Interleaver

Inner Coder

Outer

Coder

conv. / turbo

1/2 rate

variable

Puncturer

RS(204,188)

Outer

conv.

Interleaver

Inner Coder

Outer

Coder

conv. / turbo

1/2 rate

variable

Puncturer

MPEG-2

source

coder

MPEG-2

source

coder

Scrambler

Figure1: SchematicoftheDVBterrestrialtransmitterfunctions.

GMT

Jan 22 16:02

1301

0.0

0.2

0.4

0.6

0.8

1.0

Amplitude

0

5

10

15

20

Time delay [

µ

s]

Figure 2: COST 207hilly terrain (HT) type impulse

re-sponse.

cantbits[6 ](LSBs). For16-QAMand64-QAM,theupper

2bits ofeach4-or6-bit symbolwill containthemore

im-portant videodata. The lowerpriority data will thenbe

multiplexedto the lower signicance 2 bits and 4 bits of

16-QAMand64-QAM,respectively.

Rate 1=2

Inputblocklength 17952bits

Interleaver random

Numberofiterations 8

ConstraintLength 3

k 1

n 2

Polynomials 7,5

Table 2: Parameters of the inner turbo encoder used to

replacetheDVB-Tsystem'sconvolutionalcoder.

BesideimplementingthestandardDVB-Tsystemas a

benchmarker, we have improved the system by replacing

the convolutional coder by aturbo codec [8 ]. The turbo

codec'sparametersusedintheexperimentaredisplayedin

Table2. Letusnowbrieyconsiderthemultipathchannel

modelusedinourexperiments.

Video

Partitioner

FEC

Modulator

B

1

2

3

4

Input

Video

Output

High Priority Branch

Low Priority Branch

Figure3:VideopartitioningschemefortheDVB-Tsystem

operatinginhierarchicalmode.

Inthesystemcharacterisedhere,wehaveusedacarrier

frequencyof500MHzand asamplingrateof7/64s. The

channelmodelemployedinthisstudywasthetwelve-path

COST 207 [9] hilly terrain (HT) type impulse response,

withamaximalrelativepathdelayof19.9 s. Eachofthe

pathswas fadedindependentlyobeying aRayleighfading

distribution,according toanormalised Doppler frequency

of10

5

. Thiscorrespondstoaworst-casevehicular

veloc-ity of about200 km/h. Theunfaded impulse response is

depictedinFigure2. Inorderto facilitate un-equalerror

protection,letusnowconsider,howtopartitionthevideo

datastream.

3. PERFORMANCE OFDATAPARTITIONING

Theschematicofthehierarchicaldatapartitioningscheme

employed inseen inFigure 3. Let us refer to the equally

split rate-1/2 convolutional coded high and low priority

scenario as Scheme1. Furthermore,the rate-1/3

convolu-tional codedhighpriority dataandrate-2/3convolutional

codedlow priority data based scenario is referred to here

asScheme2.Lastly,therate-2/3convolutionalcodedhigh

priority data and rate-1/3 coded low priority data based

partitioning scheme is termed as Scheme 3. The

associ-atedso-calledprioritybreakpoint[1 ](PBP)histogramsare

showninFigures 4(a),5(a) and 6(a). Comparing the

his-tograms inFigure4(a),5(a) andFigure6(a), weobserved

that asexpected,Scheme3hadthemostdatainthe high

priority partition,followedbySchemes1and2.

Wethenembarkedonquantifying theerrorsensitivity

of thepartitioning Schemes1to3,whensubjectedto

ran-domlydistributedbiterrors. ThePeakSignal-to-noise

(3)

histogramR12_R12.gle

2 3 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84

Priority Breakpoint

0.0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Occurence

Frequency

(%)

partition_error_R12_R12.gle

10

-5

2

5

10

-4

2

5

10

-3

2

5

10

-2

2

BER

0

5

10

15

20

25

30

A

verage

PSNR

degradation

(dB)

Only low priority partition in error

Only high priority partition in error

Rate1/2 - Rate1/2 pair

Figure4: (a)Histogramoftheprobabilityofoccurencefor

variouspriority breakpointsand (b)averagePSNR

degra-dation versus BER for rate-1/2 convolutional codedhigh

andlowprioritydatainScheme1.

wasdenedas:

PSNR=10l og

10 P

N

n=0 P

M

m=0 255

2

P N

n=0 P

M

m=0

2

(1)

whereisthe dierencebetweenthe uncodedpixelvalue

andthereconstructedpixelvalue.

Specically, theaverage PSNRdegradation was

evalu-ated for given error probabilities inicting random errors

imposed ononeof thepartitions, while keepingthe other

partition error-free. These results are portrayed in

Fig-ures4(b),5(b) and6(b), for Schemes 1to3. Due to lack

of space the interpretation of these results is left for the

reader. Inthe nextsection we will provideoverallsystem

performanceresults.

4. PERFORMANCE OFTHEDVB

TERRESTRIALSCHEMEEMPLOYING

HIERARCHICAL TRANSMISSION

Below we will invoke the DVB-T hierarchical scheme in

a mobile broadcasting scenario. We shall also show the

improvementswhichturbocodesoer,whenreplacingthe

histogramR13_R23.gle

2 3 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84

Priority Breakpoint

0.0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Occurence

Frequency

(%)

10

-5

2

5

10

-4

2

5

10

-3

2

5

10

-2

2

BER

0

5

10

15

20

25

30

A

verage

PSNR

degradation

(dB)

degra-dation versus BER for the rate-1/3 convolutional coded

highprioritydataandrate-2/3convolutionalcodedlow

pri-oritydatainScheme2.

convolutionalcodeinthestandardscheme. Hence,the

con-volutionalcodecinboththehighandlowprioritypartitions

was replaced by the turbo codec. We have also

investi-gatedreplacing onlythehigh priority convolutionalcodec

withthe turbocodec,pairing the 1/2-rateturbocodecin

the high priority partition with the convolutional codec

inthe low priority partition. Sucha hybrid arrangement

wouldconstituteareduced-complexitycompromisescheme.

Again, the "Football" sequencewas used inthese

experi-ments.

Theperformanceoftheinvestigatedmobilevideo

broad-castsystemischaracterisedwiththeaidofFigures7and8,

theexplorationofwhichisleftforthereaderduetolackof

space. Aspecicproblemfaced,whenusingthedata

parti-tioningschemeinconjunctionwiththehighpriority

parti-tionbeing protectedbytherate1/2codeandthelow

pri-oritypartitionprotectedbytherate3/4and7/8codeswas

that when the low priority partition data was corrupted,

theerror-freehighprioritydataavailablewasinsuÆcientfor

concealingtheerrors. Wehavealsoexperimentedwiththe

(4)

histogramR23_R13.gle

2 3 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84

Priority Breakpoint

0.0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Occurence

Frequency

(%)

10

-5

2

5

10

-4

2

5

10

-3

2

5

10

-2

2

BER

0

5

10

15

20

25

30

A

verage

PSNR

degradation

(dB)

degra-dation versus BER for the rate-2/3 convolutional coded

highprioritydataandrate-1/3convolutionalcodedlow

pri-oritydatainScheme3.

convolutionalcoding,inorderto protectthehighandlow

prioritydata, respectively. FromFigure 8(a) weobserved

thattheperformanceofthis combinationapproachedthat

oftherate1/2convolutionalcodeinbothpartitions. This

wasexpected,sincenowmoredatacanbeinsertedintothe

highprioritypartition. Hence,intheeventofdecoding

er-rors inthe low priority data we had more error-freehigh

prioritydatathat canbe usedtoreconstruct thereceived

image.

Our last combination investigatedinvolved using rate

1/2turbocodingandconvolutionalcodingforthehigh-and

low-priority partitions, respectively. Comparing Figures 9

and8(a),thechannelSNRrequiredforachievingerrorfree

transmissioninbothcasesweresimilar. Thiswasexpected,

sincetheturbo-convolutionalcombination'sperformanceis

dependentontheconvolutionalcode's performanceinthe

lowprioritypartition.

Lastly,comparingFigures8and10, wefoundthatthe

error-freeconditionwasachievedat similarchannelSNRs

suggestingthatthe datapartitioning schemehadnot

pro-videdsuÆcient performance improvements inthe context

wideband_hier_RS_conv_code_BER_vs_Eb_over_No.gle

0

5

10

15

20

25

30

35

Eb/No (dB)

10

-6

10

-5

10

-4

10

-3

10

-2

10

-1

10

0

BER

64QAM Guard 1/4

16QAM Guard 1/4

Conv R7/8 (Low Priority)

Conv R2/3 (High Priority)

Conv R1/2 (High Priority)

(a)RSandConvolutionalCode

wideband_hier_RS_turbo_code_BER_vs_Eb_over_No.gle

0

5

10

15

20

25

30

35

Eb/No (dB)

10

-6

10

-5

10

-4

10

-3

10

-2

10

-1

10

0

BER

64QAM Guard 1/4

16QAM Guard 1/4

Turbo R1/2 (Low Priority - 2 iterations)

(b)RSandTurboCode

Figure 7: BER after (a) RS and convolutional decoding

and(b)RSandturbodecodingfortheDVB-Thierarchical

schemeoverthewidebandfadingchannelofFigure2using

theschematicofFigure3.

of the mobile DVB scheme, in order to justify itsadded

complexity.

5. CONCLUSIONS ANDFUTUREWORK

Inthis paper, we have investigated the performance of a

turbo-codedDVBsysteminamobileenvironment. Arange

ofsystemperformance results was presented basedonthe

standardschemeaswellasonaturbo-codedscheme. The

convolutional code specied in the standard system was

substituted with turbo coding, which resulted in a

sub-stantialcodinggainofaround5dB,whichcanbeinvested

inemployinga higher-throughput,lessrobustmodulation

mode, suchas 16QAM. Thisthenallows us todoublethe

associated bitrate within a given bandwidth, resulting in

improvedvideo quality. We have also applieddata

parti-tioningtotheMPEG-2videostreaminordertogauge its

(5)

psnr_vs_Eb_over_No_wideband_hier_RS_conv.gle

10

15

20

25

30

35

40

Channel SNR (dB)

5

10

15

20

25

30

A

verage

PSNR

(dB)

64QAM Guard 1/4

16QAM Guard 1/4

CONV R2/3 (High Priority) CONV R1/2 (Low Priority)

error free

(a)StandardScheme

psnr_vs_Eb_over_No_wideband_hier_RS_turbo_turbo.gle

10

15

20

25

30

35

40

Channel SNR (dB)

5

10

15

20

25

30

A

verage

PSNR

(dB)

64QAM Guard 1/4

16QAM Guard 1/4

Turbo R1/2 (High and Low Priority - 4 iterations)

Turbo R1/2 (High and Low Priority - 8 iterations)

error free

(b)TurboCodeinbothpartitions

Figure8: AveragePSNRversuschannelSNRfor(a)

stan-dard DVB scheme [1 ] and (b) system with turbo coding

employedinbothpartitions,fortransmissionoverthe

wide-bandfadingchannelof Figure2for hierarchical

transmis-sionusingtheschematicofFigure3.

psnr_vs_Eb_over_No_wideband_hier_turbo_conv.gle

10

15

20

25

30

35

40

Channel SNR (dB)

5

10

15

20

25

30

A

verage

PSNR

(dB)

64QAM Guard 1/4

16QAM Guard 1/4

Turbo R1/2 (High Priority) CONV R1/2 (Low Priority)

error free

Figure9: AveragePSNRversuschannelSNRoftheDVB

scheme,employingturbocodinginthehighpriority

parti-tionandconvolutionalcodinginthelowprioritypartition,

overthewidebandfadingchannelofFigure2for

hierarchi-caltransmissionusingtheschematicofFigure3.

psnr_vs_Eb_over_No_wideband_nonhier.gle

0

5

10

15

20

25

30

35

40

45

Channel SNR (dB)

10

15

20

25

30

A

verage

PSNR

(dB)

64QAM Guard 1/4

16QAM Guard 1/4

QPSK Guard 1/4

CONV R7/8

CONV R1/2

TURBO R1/2

error free

2dB degradation

Figure10: AveragePSNRversuschannelSNRoftheDVB

scheme[1]overthewidebandfadingchannelofFigure2for

non-hierarchicalmobiletransmission.

codec. However,fromtheseexperimentswefoundthatthe

data partitioning scheme did not provide substantial

im-provements comparedto the non-partitionedvideo

trans-mittedoverthenon-hierarchicalDVB-Tsystem.Ourfuture

workwillbefocusedonimprovingthesystem'srobustness

by invokinga range of so-called maximum-minimum

dis-tance Redundant Residue NumberSystem(RRNS) codes

andturboBCHcodes. FormoredetailonDVBthe

inter-estedreaderisreferredto[10 ].

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struc-ture,channel codingandmodulationfor11/12GHz

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[9] M.Failli,\DigitallandmobileradiocommunicationsCOST

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[10] L.Hanzo, P.J. Cherriman, and J. Streit, Modern Video

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